# EIRP to ERP Calculator

Use this tool to find the ERP (Effective Radiated Power) from EIRP (Effective Isotropic Radiated Power).

ERP to EIRP

## Formula

ERP = EIRP/1.64

ERPdBm = EIRPdBm – 2.15

## Background

### What is the difference between EIRP and ERP?

EIRP is the power that would have to be radiated by an isotropic antenna to provide the same signal level as the actual source antenna in the direction of the antennaâ€™s strongest beam.

ERP is the power that would have to be radiated by a half-wave dipole antenna to provide the same signal level as the actual source antenna in the direction of the antennaâ€™s strongest beam.

In linear terms, the ratio of the gain of an isotropic antenna to that of a half-wave dipole antenna is given by the following relationship:

GI/GD = 1.64

Therefore on the Log scale 10*Log10(GI/GD) = 10*Log10(1.64)

or

EIRP (dBm) – ERP (dBm) = 2.15 dB

### Applications

FCC part 15 stipulates the EIRP limits in various frequency bands. Use the calculator on this page to convert the limits to those for ERP.

Wi-Fi equipment manufacturers will use RP-SMA connectors on Access Points. This is done to prevent customers from using high gain antennas (most of which have regular SMA connectors) to increase the EIRP which will in turn exceed the FCC limits.